Fast and Furious: Longer-lasting Electric Motorcycle 16S-17S Lithium-ion Battery Pack

With the rapid growth in demand for express delivery services, electric motorcycles are becoming increasingly popular due to their advantage of having a larger battery capacity than electric bicycles and electric scooters. The larger the capacity, the longer the driving time, which helps save time and enables longer-distance deliveries.

Electric motorcycle battery packs have multiple voltage platforms, the most common of which is 60V, which requires 16S or 17S lithium-ion batteries in a battery pack.

Achieving longer runtimes requires solving three design challenges:

• High-precision battery voltage sampling to improve battery capacity calculation accuracy.

• Cell voltage balancing.

• Low system current consumption, especially in standby mode.

The low current loss 16S-17S battery pack reference design can help solve the design challenges mentioned above. It uses the BQ76940 battery monitor for the battery pack's lower 15-string battery voltage sampling and monitoring, and uses a dual-channel general-purpose operational amplifier LM2904B to monitor the upper two-string battery voltage. A larger battery balancing current is achieved through an external metal oxide semiconductor field effect transistor (MOSFET). The block diagram of the battery pack reference design is shown in Figure 1.

Figure 1: 16S-17S battery pack block diagram

The BQ76940 directly monitors the lower 15 cells, so the lower 15 cell voltage accuracy is directly determined by the BQ76940. The typical voltage sampling accuracy from 3.2 V to 4.6 V at 25°C is ±15 mV. If necessary, its voltage sampling accuracy can be further improved by additional calibration. The discrete circuit shown in Figure 2 determines the accuracy of the two upper cells.

Figure 2: Discrete circuit diagram of the two upper batteries

Take 17 strings of batteries as an example. When Q25 works in linear mode, one channel of the LM2904B works together with the P-channel MOSFET Q25, R89 and R96 as a negative feedback circuit. The negative input voltage of the amplifier is equal to the positive input voltage, that is, the voltage of the 16 strings of batteries. Therefore, the current generated by the voltage of the 17th string of single cells added to both ends of R89 flows through Q25 and R96 and returns to the reference ground. The sampling of the 16th string of single cells is similar.

By measuring the ADC_16 and ADC_17 voltages using an analog-to-digital converter (ADC), the voltages of the 16th and 17th strings of cells can be monitored. Considering the tolerances of R89, R96, R87, R94, and the ADC reference, a two-point calibration is required to achieve higher accuracy. Figure 3 shows the process of the two-point calibration.

Figure 3: Two-point calibration process

I tested the voltage accuracy of the 16th and 17th battery strings after calibration in the lab; the results are shown in Figure 4. The accuracy is ±2mV.

Figure 4: 16- and 17-cell battery voltage accuracy (at 25°C)

Since the 16th and 17th strings are monitored by discrete circuits, while the lower 15 cells are monitored by the BQ76940, the impact on cell balancing must be considered.

Figure 5 shows the main current paths. Red indicates the power supply path of the general-purpose operational amplifier, green indicates the voltage sampling path of the 17th string of batteries, and gray indicates the sensing path of the 16th string of batteries. The power supply current of the general-purpose operational amplifier is provided by the entire battery pack and flows back to the reference ground, so it discharges the entire battery pack and does not cause imbalance. The voltage sampling path of the 17th string of batteries also flows back to the reference ground from the entire battery pack, so it does not cause imbalance. However, the voltage sampling path of the 16th string of batteries flows back to the reference ground from the lower 16 strings of batteries, which will cause voltage imbalance between the 17th string and the lower 16 strings of batteries. This imbalance only occurs when the voltage of the 16th string of batteries is detected.

To reduce the impact of imbalance, Q21 can be turned off when the 16th battery string is not detected, and the Q21 control circuit current can be considered when calculating the impact of imbalance.

Based on the analysis here, and assuming a voltage sampling period of 250ms, the unbalanced current for this reference design should be less than 0.1 µA.

Figure 5: Discrete circuit current path diagram

In a previous article, “Pedal Power Solutions: 13S, 48V Li-Ion Battery Packs with Better Endurance for E-Bikes and E-Motorcycles ,” I explained how to use the LM5164 and system-level design to reduce system-level current consumption in standby mode. Now, I want to briefly discuss how to reduce the current consumption of discrete circuits in standby mode. Standby mode is neither charging nor discharging. Battery voltage sensing plays a protective role, usually by increasing idle time to reduce frequency. To reduce power consumption in standby mode, you can shut down circuits when voltage sensing is not required.

The solution in Figure 2 uses a P-channel MOSFET Q20 to switch power to the LM2904B and is controlled by a microcontroller. To further reduce the current, I added Q22 and Q21 to cut off the battery voltage sensing line, saving more energy. Assuming a voltage sensing period of 250 ms and an idle time of 250 ms, the average current consumption in standby will be quite low. The typical current in the solution shown in Figure 2 is less than 1 µA.

Overall, the reference design provides a cost-competitive battery pack solution covering up to 17S batteries, ideal for electric motorcycles. The design achieves longer runtimes by:

• Improve battery voltage sampling accuracy.

• Reduce current consumption in standby mode.

• Eliminate imbalances.

This design is also suitable for telecom backup battery packs that require 16S/48-V lithium-ion phosphate battery packs.